In eukaryotes, protein degradation is predominately mediated through the ubiquitin pathway in which proteins targeted for destruction are ligated to the 76 amino acid polypeptide ubiquitin. Once targeted, ubiquitinated proteins then serve as substrates for the 26S proteasome, a multicatalytic protease, which cleaves proteins into short peptides through the action of its three major proteolytic activities. While having a general function in intracellular protein turnover, proteasome-mediated degradation also plays a key role in many processes such as major histocompatibility complex (MHC) class I presentation, apoptosis, cell division, and NF-κB activation.
The 20S proteasome is a 700 kDa cylindrical-shaped multicatalytic protease complex comprised of 28 subunits organized into four rings that plays important roles in cell growth regulation, major histocompatibility complex class I presentation, apoptosis, antigen processing, NF-κB activation, and transduction of pro-inflammatory signals. In yeast and other eukaryotes, 7 different α subunits form the outer rings and 7 different β subunits comprise the inner rings. The α subunits serve as binding sites for the 19S (PA700) and 11S (PA28) regulatory complexes, as well as a physical barrier for the inner proteolytic chamber formed by the two β subunit rings. Thus, in vivo, the proteasome is believed to exist as a 26S particle (“the 26S proteasome”). In vivo experiments have shown that inhibition of the 20S form of the proteasome can be readily correlated to inhibition of 26S proteasome. Cleavage of amino-terminal prosequences of β subunits during particle formation expose amino-terminal threonine residues, which serve as the catalytic nucleophiles. The subunits responsible for catalytic activity in proteasome thus possess an amino terminal nucleophilic residue, and these subunits belong to the family of N-terminal nucleophile (Ntn) hydrolases (where the nucleophilic N-terminal residue is, for example, Cys, Ser, Thr, and other nucleophilic moieties). This family includes, for example, penicillin G acylase (PGA), penicillin V acylase (PVA), glutamine PRPP amidotransferase (GAT), and bacterial glycosylasparaginase. In addition to the ubiquitously expressed β subunits, higher vertebrates also possess three γ-interferon-inducible β subunits (LMP7, LMP2 and MECL1), which replace their normal counterparts, X, Y and Z respectively, thus altering the catalytic activities of the proteasome. Through the use of different peptide substrates, three major proteolytic activities have been defined for the eukaryote 20S proteasome: chymotrypsin-like activity (CT-L), which cleaves after large hydrophobic residues; trypsin-like activity (T-L), which cleaves after basic residues; and peptidylglutamyl peptide hydrolyzing activity (PGPH), which cleaves after acidic residues. Two additional less characterized activities have also been ascribed to the proteasome: BrAAP activity, which cleaves after branched-chain amino acids; and SNAAP activity, which cleaves after small neutral amino acids. The major proteasome proteolytic activities appear to be contributed by different catalytic sites, since inhibitors, point mutations in β subunits and the exchange of γ interferon-inducing β subunits alter these activities to various degrees.
In recent years, the proteasome has become an appealing target for therapeutic intervention in cancer, immune and auto-immune disorders, inflammation, ischemic conditions, neurodegenerative disorders and other diseases. To date, the only FDA-approved proteasome inhibitor is bortezomib (VELCADE™), however, several other proteasome inhibitors are currently being evaluated in clinical trials. Thus far, all these therapeutic proteasome inhibitors currently are administered via IV. Clinical application of proteasome inhibitors in the treatment of hematologic malignancies such as myeloma and lymphoma is restricted in part by the necessity of frequent IV administrations and would be improved by oral (PO) administration. However, due to the peptide nature of these molecules, systemic exposure following PO administration of these inhibitors is limited by several factors including gastric pH, gastric and intestinal peptidases, efflux pumps, biliary excretion and intestinal and hepatic metabolic activities.
Methods used to overcome the ability of peptides to be enzymatically degraded and to improve absorption into the blood stream from the digestive tract have included making analogs which are less peptide-like in structure and which are reduced in size. Such methods are deemed to be successful when the peptide analog achieves satisfactory blood levels after oral administration, or in the case of proteasome inhibitors, when the proteasome activity in blood is satisfactorily reduced.
The above mentioned techniques have been applied to preparing analogs of the peptide epoxyketone proteasome inhibitors, thereby rendering them orally bioavailable.